The present invention relates to a gas-pressure relief valve unit, particularly for fuel vapour, comprising a body having means adapted to allow its connection in correspondence of an opening of a top wall of a fuel tank, the body defining a chamber adapted to communicate with first and second environments, upstream and downstream of the unit respectively, through first and second apertures, first and second valve means being associated with the body, the first valve means being associated with the chamber and including a first valve seat and an obturator movable between a position of engagement with said first seat, in which the first valve means cut off at least part of a vapour flow between the apertures, and a position spaced from the first seat, the obturator being urged by a resilient thrust element towards its position of engagement with the first seat, whereby it can be moved away from the first valve seat, against the action of the resilient element, when a predetermined threshold value of the pressure differential between the environments is reached, in the position in which the obturator is engaged with the first seat and when the pressure differential between the environments is less than the predeterminated threshold value, the first valve means define a narrow passage with respect to said first and second apertures such as to allow a small, continuous flow of gas between the first and second environments, the second valve means comprising a second valve seat.
A valve unit of the type defined above is known, for example, from Italian Patent Application No. TO92U000198. This known unit, intended for application to a wall of a vehicle fuel tank, includes a pressure relief valve complex constituted by two one-way flow valves operating in opposite directions.
More particularly, a first valve, having an annular seat and a metal disc obturator urged towards its seat by a spring enables the interior of the tank to be put in communication with the exterior when the pressure within the tank exceeds a predetermined value.
The second valve, constituted by a disc of elastomeric material supported by the disc obturator itself and having a central hole from which projects a pair of separable lips which normally mate together, enables the external environment to be put in communication with the interior of the tank when the pressure in the tank falls below a predetermined value relative to the external pressure.
The production of such a valve complex is however relatively complicated and thus expensive. It is in fact necessary to provide two valves and then to assemble several elements together in a restricted space. In addition, these elements are usually of different materials from each other, such as metal and elastomer, which are in turn different from the material forming the body of the unit. More particularly, the fact that one of the one-way flow valves includes at least one elastomeric element, as is preferred to ensure the desired flexibility and elasticity of the pair of separable lips, may be critical with regard to the reliability of operation of the entire unit. It is in fact known that elastomers age in the presence of hydrocarbons and lose their properties of flexibility and elasticity in time.
This known unit is also rather noisy, in use, as each of the one-way flow valves is subject to a continuous cycle of opening and closing. Indeed, each of these valves opens only when the pressure differential between the gases in the environments upstream and downstream exceeds a predetermined threshold. As a result, each one-way flow valve, at least in the instants immediately after its opening, experiences a relatively high rate of flow of gas due to the flow of the quantity of gas which has previously accumulated under pressure in the respective environment upstream or downstream. In particular, the obturator knocks against the seat of the first one-way flow valve, which normally works in a condition of unstable equilibrium, assuming its open and closed configurations in succession at a high frequency. The second valve, the maximum opening of which is limited by the fact that it is disposed radially within the first valve, also operates by allowing successive quantities of gas through as these reach a pressure sufficient to open it and, since the gas usually passes through at a relatively high velocity, this may cause whistling noises as a result of the vibrations in the separable lips.
A valve unit of the type defined above is also known from document U.S. Pat. 5,028,244.